Recently, OFDM (Orthogonal Frequency Division Multiplex: orthogonal frequency division multiplexing) system has been employed as a transmission system highly resistant to multi-pass interference as well as having an excellent frequency use efficiency. According to this OFDM system, a plurality of carriers (hereinafter referred to as sub-carrier) orthogonal to each other are disposed at every predetermined frequency interval within a single transmission band and data is distributed to respective sub-carriers and modulated for transmission. According to this system, its transmission apparatus disposes transmission data obtained in time series virtually on a frequency axis, allocates transmission data to each sub-carrier, and orthogonally transforms it to multi-carrier signals at the predetermined frequency interval by reverse fast Fourier transformation or the like. A receiving apparatus thereof converts received multi-carrier signal inversely to transmission time to data secured in time series so as to obtain reception data.
FIG. 1 is a diagram showing an example of the structure of a radio transmission apparatus according to the OFDM system. Hereinafter, the structure will be described. Here, a radio transmission apparatus 100 comprises a video circuit 101 and a voice circuit 102 and the respective circuits 101, 102 carries out processing for encoding the inputted video signal and voice signal. For example, the video circuit 101 performs encoding according to irreversible image compression encoding method such as a processing for converting animation video signal to MPEG (Moving Picture Expers Group) system image data, a processing for converting static image video signal to JPEG (Joint Photographing coding Expers Group) system image data. Or encoding with reversible image compression method like JBIG (Joint Bi-level Image Experts Group) is permissible. A voice circuit 102 carries out encoding based on the MPEG audio method, CELP (Code Excited Linear Prediction) method, PCM (Pulse Code Modulation) method or the like. In the meantime, the coded data may be provided with ECC (Error Correcting Code) such as the Reed-Solomon code, the turbo code and the like.
Video data outputted by a video circuit 101 and voice data outputted by the voice circuit 102 are supplied to a mixing circuit 103 in which they are converted to single-system data. After that, it is supplied to an interleaver 104, in which interleave processing is carried out by changing data arrangement to disperse bit series. Data interleaved by the interleaver 104 is subjected to modulation processing by a modulator 105. In this modulator 105, first, a preamble signal is inserted into the bit sequence and next, as a primary modulation, for example, DQPSK modulation (Differential Quadrature Phase Shift keying) is carried out. In the meantime, other modulation method than the DQPSK modulation may be employed such as QPSK, BPSK, 8PSK, QAM and the like.
Data primarily modulated by the modulator 105 is supplied to a reverse fast Fourier transformation circuit (IFFT circuit) 106 and as a secondary modulation, reverse Fourier transformation processing for converting data disposed on time axis to data arrangement on the frequency axis by arithmetic processing of reverse Fourier transformation is carried out and further, window application processing is carried out by multiplying window data. If the reverse Fourier transformation processing is carried out in this IFFT circuit 106, a transmission symbol stream disposed on the frequency axis virtually up to then is averaged so as to be transmission series. In the IFFT circuit, each time when data of a predetermined unit is inputted, reverse Fourier transformation arithmetic processing is carried out for that inputted data. In this specification, time for carrying out the arithmetic processing of this one unit is called single modulation time.
Output of the IFFT circuit 106 is supplied to a digital/analog converter 107 so as to be converted to analog signal. After the conversion, the analog signal is supplied to a high-frequency portion (RF portion) 108, in which high-frequency processing such as filtering, frequency conversion are carried out so as to gain a transmission signal of a predetermined transmission channel. After that, it is transmitted by radio through an antenna 110. Processing timing in each circuit in the radio transmission apparatus 100 is controlled by a time base controller (TBC) 109.
FIG. 2 is a diagram showing a radio reception apparatus for receiving a signal transmitted from the radio transmission apparatus 100 shown in FIG. 1. Hereinafter, the structure thereof will be described. The radio reception apparatus 200 supplies a signal received by an antenna 201 to a high-frequency portion (RF portion) 202 so as to carry out such reception processing as filtering and frequency conversion. Consequently, a reception signal of a predetermined channel is obtained. This reception signal is supplied to the analog/digital converter 203 and converted to digital data. Reception series subjected to digital conversion is supplied to a window detecting portion 204. This window detecting portion 204 carries out processing for detecting synchronism by detecting a break in data to be subjected to Fourier transformation based on window data multiplied by the transmission system from reception series.
Output of the window detecting portion 204 is supplied to the fast Fourier transformation circuit (FFT circuit) 205 and transformation processing is carried out, in which Fourier transformation action is carried out at the timing of the break in data detected by the window detecting portion 204 and data on the frequency axis is converted to data arrangement on time axis by the arithmetic processing of the Fourier transformation. The reception series Fourier transformed is supplied to a decoder 206, in which decoding processing for returning conversion processing applied at the time of transmission such as the DQPSK modulation is carried out so as to generate a reception symbol stream.
This reception symbol stream is supplied to a deinterleaver 207, in which deinterleave processing for returning bit series dispersed by interleave processing at the time of transmission to its original data arrangement is carried out so as to obtain reception encoding bit series. This reception encoding bit series is supplied to a viterbi decoder 208 and converted to reception information bit series by viterbi decoding processing. Video information in the converted reception information bit series is supplied to a video circuit 209 and voice information is supplied to a voice circuit 210.
In the video circuit 209, data encoded by the video circuit 101 of the transmission system is decoded so as to obtain transmitted video data. In the voice circuit 210, data encoded by the voice circuit 102 of the transmission system is decoded so as to obtain transmitted voice data. Processing timing in each circuit in the radio reception apparatus 200 is controlled by a time base controller (TBC) 211.
With the above described structure, transmission and reception of the OFDM system signal are carried out. The primary modulation by the modulator 105 at the time of transmission is a modulation system in which the phase of carrier is changed in discrete manner depending on transmission data, so that it has a large advantage in frequency application efficiency. Because, in the reverse Fourier transformation processing in the IFFT circuit 106, the bit series disposed on the subcarrier is averaged on time axis, it has such a large advantage that it is highly resistant to interfering wave such as fading and shadowing.
However, on the side receiving such multi-carrier signal, respective bit seriess cannot be decoded until Fourier transformation processing in the FFT circuit 205 is carried out. If a break for one modulation (hereinafter referred to break) is not recognized properly when the FFT circuit 205 executes the Fourier transformation processing at the time of reception, accurate bit series cannot be decoded.
To achieve proper Fourier transformation action in the FFT circuit, it is necessary to determine the break depending on a power level of transmission data because the break (hereinafter referred to as break) of one modulation time cannot be determined from data received in a circuit on a prestage of the Fourier transformation circuit (window detecting portion 204 in FIG. 2). Ordinarily, for the known preamble signal contained in the transmission data, correlation in power level is obtained. In order to increase the accuracy of correlation value to be obtained here, calculation is necessary without reducing the bit width of each channel. For the reason, there is a problem that the scale of a circuit for detecting the correlation is increased.